Manufacture of state of the art multimode optical fiber requires demanding control over a variety of power loss and signal impairment mechanisms. For multimode fiber, controlling mode dispersion is an important goal.
As is well known, within the optical fiber, bits of data are represented by pulses of light. Each pulse of light will spread, or disperse, over time as it travels the length of the fiber. If these data pulses overlap, they can no longer be unambiguously read at the receiving end. Lower tendency toward data pulse overlap results in higher data transmission capacity, i.e. higher bandwidth. Therefore, the bandwidth of optical fibers is ultimately limited by dispersion.
Predominant forms of dispersion are chromatic dispersion and mode dispersion. Chromatic dispersion is well known and occurs is all optical fiber systems. Mode dispersion, or intermodal dispersion, occurs mainly in multimode optical fibers where the large core diameter allows a wide number of optical paths for light to travel. Different optical paths usually have different lengths. Because the modes travel along paths of varying length, they arrive at the fiber end at different intervals of time. If the time difference is great enough, the pulse traveling the faster path will overlap the pulse ahead of it.
Multimode optical fiber bandwidth is optimized by minimizing intermodal dispersion. This is commonly achieved by using graded index profiles wherein the refractive index gradually increases from the outer region of the cladding to the center of the core. Signals travel faster in the low-index region near the cladding, and slower in the high-index region near the center of the core.
In multimode optical fiber, mode dispersion may be referred to as Differential Mode Delay (DMD). Manufacturing specifications for optical fiber for use in state of the art systems have rigid requirements for DMD. The DMD specification within a given optical fiber core radius is called mask width. For example, fibers with DMD of less than 0.23 ps/m within a core radius of 18 microns are referred to as having a mask width within the 18 micron radius of <0.23 ps/m. This may also be expressed as MW 18<0.23 ps/m. These mask width specifications correspond to an optical fiber having an Effective Modal Bandwidth (EMB) of 2000 MHz-km at 850-nm, and optical fibers meeting these mask width specifications typically have overfilled bandwidths >500 MHz-km at 1300-nm. 850-nm and 1300-nm are typical wavelengths of choice for multimode optical systems. Manufacturing multimode optical fiber to meet these specifications has proven difficult.
It is known that short range refractive index variations, or perturbations, cause mode mixing, which has the effect of averaging the transmission distance for all modes traversing the optical fiber. Techniques for enhancing mode mixing have been sought by workers in the art to address adverse DMD. One of these is described in U.S. patent application Ser. No. 847,034 filed May 1, 2001 by DiGiovanni et al., which is incorporated by reference herein in its entirety. In that approach the fiber core is made non-circular, and the optical fiber is twisted during draw. Significant increases in bandwidth result. However, there is always a quest for further improvements in multimode optical fiber bandwidth.